A pacemaker is a medical device for implant within a patient that recognizes various arrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmias. An ICD is a device, also for implant within a patient that additionally recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate the fibrillation.
Pacemakers and ICDs carefully monitor characteristics of the heart such as the heart rate to detect arrhythmias, discriminate among different types of arrhythmias, identify appropriate therapy, and determine when to administer the therapy. The heart rate is tracked by examining electrical signals that result in the contraction and expansion of the chambers of the heart. The contraction of atrial muscle tissue is a result of the atrial depolarization or electrical activation of the atrial tissue manifested as a P-wave in a surface electrocardiogram (ECG). The EGM is a recording of the electrical signal from within the heart and, in the case of the atrium, is referred to as an atrial EGM. The contraction of ventricular muscle tissue follows the electrical depolarization of the ventricle, which is manifest on the ECG by an R-wave (sometimes referred to as the “QRS complex”) and as sharp deflection within a ventricular EGM termed the intrinsic deflection. Recovery of the cardiac electrical potential is manifest as a T-wave on the ECG. With the T-wave, the active cardiac contraction ceases and the ventricle begins to relax and dilate allowing the ventricle to expand and fill with blood in preparation for the next cardiac contraction or heartbeat. A similar phase involving the atrial tissue occurs but usually does not result in a detectable signal on the ECG because it is a smaller signal proportional to the P-wave amplitude and coincides with and is obscured by the QRS complex. The sequence of electrical events that represent P-waves, followed by R-waves, followed by T-waves can be detected within EGM signals sensed using pacing leads implanted inside the heart.
One commonly used type of lead is the unipolar lead, which includes a single electrode at or near its tip. A sense amplifier detects electrical voltage differentials between the electrode and the external body or housing of the pacemaker. In a dual-chamber, dual-unipolar pacing system, one unipolar lead is inserted within the atria and another within the ventricles, from which the device derives separate atrial and ventricular channel EGM signals. A problem with unipolar leads is that, because the sense amplifier detects the voltage differential between the tip of the lead and the housing of the pacemaker, the detection antenna is large, thereby allowing the system to detect extracardiac signals and significant electrical signals from the opposite cardiac chamber (termed “far-field”) as well as the intended atrial or ventricular EGM signals. A far-field signal is a signal originating from the opposite cardiac chamber but detected by the sensing lead nonetheless, delivered to the sense amplifier within the pacemaker from the channel to which the lead is connected and potentially interpreted as arising from that chamber. For example, the atrial EGM signal derived from the atrial lead may include significant ventricular signals and, in the case of single chamber pacing systems, may be reset and recycled by this detected but inappropriate signal. Note also that the tissue mass in the ventricle is relatively large, resulting in a large electrical signal in either the bipolar or the unipolar sensing configurations. As far-field signals tend to be of low amplitude, far-field signals recordable on the ventricular channel are commonly managed by making the ventricular channel less sensitive. However, signals arising from the atrium usually have relatively low amplitudes. As such, the pacemaker must be programmed to a very sensitive setting in order to appropriately detect near-field intrinsic atrial signals, resulting in possible detection of far-field signals. Indeed, keeping the atrial channel set to a very sensitive setting predisposes the detection of far-field signals.
Another commonly employed type of lead is the bipolar lead, which includes two electrodes mounted in close proximity to one another within the heart. Usually, one electrode is called the “tip” and the other the “ring.” Typically, one bipolar lead is inserted within the atria and another within the ventricles, from which the device derives separate atrial and ventricular signals. An atrial sense amplifier detects electrical voltage differentials between the tip and ring electrodes of the atrial lead. A ventricular sense amplifier detects electrical voltage differentials between the tip and ring electrodes of the ventricular lead. The use of bipolar sensing leads improves the signal-to-noise ratio and allows the sensitivity to be set to a very sensitive setting without significant risk of inadvertent detection of extracardiac or far-field events. However, bipolar leads are more complex than unipolar leads and, based on their track record, are perceived as being less reliable than unipolar leads and hence are not preferred by all physicians. Further, even in circumstances wherein a bipolar lead is employed, it may need to be operated in a unipolar mode. For example, if a mechanical problem occurs within a bipolar lead, the lead may need to be operated in a unipolar mode. Hence, far-field sensing problems may arise even in circumstances where bipolar leads are implanted.
In an attempt to provide the advantages of bipolar sensing using simpler unipolar leads, some state-of-the-art devices employ a combined unipolar/bipolar sensing technique, referred to as Combipolar sensing. (“Combipolar” is a trademark of St. Jude Medical.) With Combipolar sensing, unipolar leads are positioned within the heart, one in the atria and one in the ventricles. A ventricular channel EGM signal is generated in the same manner as with conventional unipolar sensing wherein electrical voltage differentials are detected between the tip of the ventricular lead and the housing of the device. However, the atrial channel of the EGM signal is generated by detecting cross-chamber voltage differentials between electrodes at the tips of the atrial and ventricular leads. A logic system internal to the pacemaker determines whether events appearing within the cross-chamber signal are atrial events or ventricular events. In this regard, an event sensed on both the atrial and ventricular channels is regarded as a ventricular event. An event sensed only on the atrial channel is regarded as a true atrial event. An event sensed only on the ventricular channel is regarded as being of extracardiac origin. For a more complete description of Combipolar systems, see U.S. Pat. No. 5,522,855 (Hoegnelid), incorporated herein by reference. Initially, the term “Combipolar sensing” was applied only to dual unipolar systems wherein the cross-chamber signal was sensed V-tip to A-tip. Herein, however, the term “Combipolar sensing” more generally applies to any system employing an atrial-to-ventricular cross-chamber signal for use in detecting atrial events, in combination with a ventricular unipolar signal for detecting ventricular events. The term “combined unipolar/bipolar sensing” is also used herein to refer to Combipolar sensing.
Thus, Combipolar sensing allows some of the advantages of bipolar sensing to be exploited within implanted able systems employing unipolar leads. However, one disadvantage is that, because the atrial signal is detected based upon voltage differentials between the tips of the atrial and ventricular leads, ventricular signals are sensed as “near-field” signals. As a result, intrinsic ventricular signals are recorded on the atrial channel. This is not a problem where the intrinsic ventricular signals are also detected on the ventricular channel since the logic of the Combipolar system will thereby regard the signals as being ventricular signals, but if an intrinsic signal arising in the ventricle is not detected on the ventricular channel but only on the atrial channel, it will be treated as a P-wave. Such may be the case with the T-wave, which may coincide with the ventricular refractory period (VRP)—a period of time when the ventricular channel does not respond to intrinsic signals (at least within some devices.)
More specifically, within a refractory period the device does not process electrical signals during a predetermined interval of time either for all device functions (an absolute refractory period) or for selected device functions (a relative refractory period). As an example, upon detection of an R-wave on the ventricular channel, a post-ventricular atrial refractory period (PVARP) is initiated on the atrial channel. Traditionally, devices were disconnected from inputs during the absolute portion of the refractory period and therefore could not sense any events. More recent devices allow sensing of atrial events throughout the PVARP, though the device does to respond to events sensed within the PVARP. Typically, a first portion of the PVARP comprises a post ventricular atrial blanking (PVAB) interval wherein the pacemaker can detect signals on the atrial channel but does not use the signals for any purpose. The PVAB is provided to prevent the device from erroneously responding to a far-field R-wave on the atrial channel. The PVARP concludes with a relative refractory period during which the pacemaker continues to ignore all signals detected on the atrial channel as far as triggering or inhibiting pacing functions is concerned, but not for other functions, such as detecting rapid atrial rates or recording diagnostic information. A total atrial refractory period (TARP) is defined as the period of time including an atrioventricular AV delay, any AV delay extension and the PVARP. The sum of the AV delay and the PVARP defines the fastest atrial rate that can be detected to still trigger a ventricular output in a 1:1 relationship.
Exemplary VRP, PVAB, PVARP and TARP periods and their affect on detecting rapid atrial rates are illustrated in FIGS. 1 and 2, which show stylized representations of a surface electrocardiogram (ECG) and atrial and ventricular EGM channels detected using Combipolar sensing. FIG. 1 illustrates the case of a normal sinus rhythm. FIG. 2 illustrates an episode of atrial tachyarrhythmia. The P-waves and R-waves appear within the atrial and ventricular IEGMs as sharp discrete complexes. The T-wave appears as a discrete signal but of lower amplitude and lower frequency response. The lower frequency components may result in this signal being effectively negated by the filters in the sensing circuit of most bradycardia pacemakers but not by those of ICDs which tend to have a broader band-pass filter to facilitate recognition of low amplitude, low frequency signals associated with ventricular tachyarrhythmias.
Referring first to FIG. 1, the PVAB interval, which begins upon detection of an R-wave on the ventricular channel, is set to a duration sufficient to cover the R-wave such that the R-wave is not detected in the atrial channel. The terminal portion of the PVARP interval that extends beyond the PVAB coincides with the T-wave. In standard Combipolar sensing algorithms, all signals detected during the terminal portion of the PVARP are ignored for the purposes of atrial rate detection. Hence, the T-wave, if capable of being detected on the atrial channel based on its amplitude and frequency content is nevertheless ignored on the atrial channel and an accurate determination of the atrial rate can be achieved, at least during normal sinus rhythm. However, during the episode of atrial tachyarrhythmia shown in FIG. 2, P-waves occurring during the terminal portion of the PVARP are also ignored along with the T-wave thereby resulting in a significant underestimate of the true atrial rate.
Accurate detection of rapid atrial rates is required, for example, for the purposes of enabling Automatic Mode Switching (AMS) algorithms wherein the pacemaker is capable of automatically switching between a tracking mode such as the VDD or DDD and a nontracking mode such as VVI or DDI mode based on whether the atrial rate exceeds an atrial tachycardia detection rate (ATDR) threshold. (Note that the term AMS is sometimes used to refer to only the actual switch from the tracking mode to the non-tracking mode. Herein the term in used to more generally refer to the capability of automatically switching back and forth between tracking and non-tracking modes based on atrial rate.) VDD, DDD, VVI and DDI are standard device codes that identify the mode of operation of the device. DDD indicates a device that senses and paces in both the atria and the ventricles and is capable of both triggering and inhibiting functions based upon events sensed in the atria and the ventricles. VDD indicates a device that sensed in both chambers but only paces in the ventricle. A sensed event on the atrial channel triggers a ventricular output after a programmable delay, the pacemaker's equivalent of a PR interval. VVI indicates that the device is capable of pacing and sensing only in the ventricles and is only capable of inhibiting the functions based upon events sensed in the ventricles. DDI is identical to DDD except that the device is only capable of inhibiting functions based upon sensed events, rather than triggering functions. As such, the DDI mode is a non-tracking mode precluding its triggering ventricular outputs in response to sensed atrial events. Numerous other device modes of operation are possible, each represented by standard abbreviations of this type. Details regarding AMS may be found in the following patents: U.S. Pat. Nos. 5,441,523; 5,591,214; 5,144,949; 4,856,523; 4,944,298; each of which is incorporated herein by reference. See also Levine et al., “Implementation Of Automatic Mode Switching In Pacesetter's Trilogy DR+ And Affinity DR Pulse Generators”, Herzschrittmacher Elektrophysiology 10 (1999) 5, S46–S57.
The atrial rate underestimate described with reference to FIG. 2 can result in a failure to trigger a mode switch to a nontracking mode in dual unipolar VDD or DDD systems or to recognize high atrial rates in a nontracking mode such as DDI. Although, nontracking devices typically do not provide for therapeutic intervention (such as AMS) because the pathologic atrial arrhythmia is not capable of being tracked, it is nevertheless desirable that the presence of the atrial arrhythmia be recognized and its duration documented for diagnostic purposes.
To ensure that the atrial rate is properly assessed, an improved Combipolar technique may be employed wherein signals detected during the terminal portion of the PVARP are used for the purposes of atrial rate calculation. In other words, P-waves detected during the relative refractory portion of the PVARP are counted in the determination of the atrial rate. However, with this improved Combipolar technique, the T-wave occurring during the terminal portion of the PVARP may also be detected on the atrial channel. Although detection of the T-wave on the atrial channel during an episode of atrial tachyarrhythmia may not be a significant problem, detection of the T-wave on the atrial channel during normal sinus rhythm will result in a significantly erroneous high atrial rate calculation. Indeed, the detected atrial rate will typically be twice the actual rate. If AMS is enabled, the erroneously high atrial rate will frequently result in unnecessary mode switching to a nontracking mode. If an atrial high rate counter is enabled, inappropriate high atrial rates may be reported.
However, standard Combipolar sensing logic, which identifies signals detected on both the atrial and ventricular channels as being of ventricular origin, would not be capable of eliminating the T-wave since, in this scenario, the T-wave is not detected on the ventricular channel because it occurs during the absolute portion of the VRP during which no sensing is permitted on the ventricular channel (at least in some devices). Even if the T-wave occurs after the absolute portion of the VRP, the T-wave typically will not be detected on the ventricular channel because of band-pass filtering on the ventricular channel or because the sensitivity of the ventricular channel is set to a less sensitive setting that effectively eliminates T-waves. In contrast, the atrial channel is usually programmed to a very sensitive setting in order to detect pathologic atrial arrhythmias, which are often of a significantly lower amplitude signal than the sinus P-wave and the near-field T-wave. Hence, even if the T-wave occurs outside the VRP, the T-wave may be detected only on the atrial channel, particularly if different filters and/or sensitivities are utilized on the atrial and ventricular channels, resulting in further problems in accurate atrial rate detection. In certain dual-chamber ICDs, the atrial and ventricular channels both employ sense amplifiers with broad band-pass filters to facilitate detection of low amplitude, low frequency fibrillatory signals. This can result in significant T-wave oversensing. Hence, even with the improved Combipolar technique, inappropriate high atrial rates may be detected.
Thus, the true atrial rate cannot reliably be determined in many dual unipolar VDD and DDD systems, even with improved Combipolar sensing techniques, and AMS is therefore not enabled usually within such devices. Hence, it would be desirable to provide improved techniques for determining the true atrial rate so as to permit the use of AMS in dual unipolar VDD and DDD systems. It would also be desirable to provide improved techniques for determining the true atrial rate in DDI device using dual unipolar leads to permit detection of high atrial rates.
In addition, when using atrial bipolar leads circumstances can arise wherein T-wave oversensing becomes an issue despite T-wave filtering. FIG. 3 illustrates exemplary EGMs along with various event markers for a normal canine sinus rhythm yet wherein an implanted dual bipolar AMS pacemaker performs an unnecessarily mode switch to a non-tracking mode due to far-field T-wave sensing on the atrial channel. (More specially, the graph illustrates a sense amp signal derived from atrial tip-ring signals along with a raw ventricular tip-ring signal.) In the example, the atrial channel is programmed to a very sensitive setting (0.3 mV) and the ventricular channel programmed to a value ten times less sensitive (3.0 mV). The PVARP is shorter than the VRP and the far-field T-wave coincides with an atrial alert period but also with the VRP. Double arrows identify some of the far-field T-waves. The far-field T-wave on the atrial channel is a very discrete and relatively sharp complex having been processed by the sensing circuit. If standard Combipolar sensing logic were to be applied to the foregoing example in an effort to avoid the unnecessary mode switch, the rhythm would still be labeled an atrial tachycardia because the T-wave was sensed on the atrial channel but not the ventricular channel and so an inappropriate mode switch to a non-tracking mode would occur. In particular, under Combipolar sensing logic, the pacemaker properly identifies event 11 as being a true P-wave because it is only detected on the atrial channel. The pacemaker properly identifies event 13 as being a true R-wave because it appears on both the atrial and ventricular channels. However, the pacemaker incorrectly identifies T-wave 15 as a P-wave because (a) it coincides with the VRP and (b) the size of the signal is smaller than the programmed sensitivity on this channel. In other words, because of T-wave filtering, the T-wave is not sensed on the ventricular channel but is only sensed on the atrial channel. Hence, T-waves are misinterpreted as P-waves causing an inappropriate mode switch.
Accordingly, it would also be desirable to provide improved techniques for implementing Combipolar sensing in atrial bipolar systems so as to avoid far-field T-wave oversensing.
Atrial rate detection problems also arise in connection with atrial leads employed in bipolar configuration if placed in close anatomic proximity to the ventricle such that a large far-field R-wave is detected. Such positions include the coronary sinus and the low interatrial septum, both locations receiving increasing attention from implanting physicians as pacing from these locations may reduce the incidence of atrial arrhythmias. Hence, even in the bipolar sensing configuration, a large far-field signal may be detected resulting in inaccurate high atrial rate detection and inappropriate mode switches. Accordingly, it would also be desirable to provide improved techniques for implementing Combipolar sensing in atrial bipolar systems so as to avoid far-field R-wave oversensing.